Section 13. Assessments made by a board of the commonwealth under this chapter shall bear interest at 1 rate of 5 per cent per annum or, at the election of the board at a rate up to 2 per cent above the rate of interest chargeable to the body politic on behalf of which the assessment was made, for the betterment project to which the assessments relate, from the thirtieth day after the date the notice of such assessments was sent by the collector. All other assessments made under this chapter shall bear interest at 1 rate of 5 per cent per annum or, at the election of the city, town or district at a rate up to 2 per cent above the rate of interest chargeable to the city, town or district for the betterment project to which the assessments relate, from the thirtieth day after the date the notice of such assessments was sent by the collector. The assessors shall add each year to the annual tax assessed with respect to each parcel of land all assessments, constituting liens thereon, which have been committed to the collector prior to January second of such year and which have not been apportioned as hereinafter provided, remaining unpaid, as certified to them by the collector, when the valuation list is completed, with interest to the date when interest on taxes becomes due and payable. At any time before the completion by the assessors of the valuation list for the year in which such assessments will first appear on the annual tax bill, the board of assessors may, and at the request of the owner of the land assessed shall, apportion all assessments or unpaid balances thereof made under this chapter into such number of equal portions, not exceeding 20, as is determined by said board or as is requested by the owner, as the case may be, but no one of such portions shall be less than 5 dollars; provided, that, if an original assessment exceeds $100 and has been placed upon the annual tax bill, or has been apportioned into a number of portions less than 20 and the first portion has been placed upon an annual tax bill, the board of assessors may in its discretion, upon a request for the apportionment of such assessment into 20 portions made by the owner prior to a sale or taking of the land for the non-payment of such assessment or portion and upon payment of any necessary intervening charges and fees and such portions of such assessment as would have become due and payable if the request for apportionment had been seasonably made, apportion or reapportion the said assessment as aforesaid, and if any other tax or assessment constituting a lien upon the parcel to which the assessment so apportioned or reapportioned relates remains unpaid after such apportionment or reapportionment, the collector may institute proceedings anew for the sale or taking of such parcel at any time prior to the expiration of the lien or of a period of 20 days after such apportionment or reapportionment, whichever is the later. In any case in which an assessment relates to a state-funded project, the apportionment or reapportionment described herein shall be undertaken in accordance with the terms aforesaid by the board on whose behalf the assessment was made; provided, however, that the apportionment shall be made of said assessments or unpaid balances together with any interest due thereon. The assessors shall add one of said portions, with interest on the amount remaining unpaid from 30 days after the date the notice of the original assessment was sent by the collector to the date when interest on taxes becomes due and payable, to the first annual tax upon the land and shall add to the annual tax for each year thereafter 1 of said portions and 1 year's interest on the amount of the assessment remaining unpaid until all such portions shall have been so added; all assessments and apportioned parts thereof, and interest thereon as herein provided, which have been added to the annual tax on any parcel of land shall be included in the annual tax bill thereon. After an assessment or a portion thereof has been placed on the annual tax bill, the total amount of said bill shall be subject to interest under and in accordance with the provisions of section 57 or section 57C of chapter 59.

Notwithstanding the foregoing, or any general or special law to the contrary, a city, town or district may elect to: (1) apportion any assessments, or the unpaid balances of such assessments, into annual portions equal to the number of years for which bonds are issued for the project for which the assessments are made; (2) structure the portions so that the amount payable each year for assessment principal and interest combined are as nearly equal as practicable or, in the alternative, provides for a more rapid amortization of the assessment principal amount where the debt service on the bonds issued for the project is so structured; or (3) make the annual portion so structured payable in the same number of preliminary and actual installments as the real estate tax in the city, town or district, with each installment equal in amount and due at the same time as each installment of the tax.

Notwithstanding a prior apportionment, the assessors, upon written application of the owner of the land assessed, shall order that the full amount, or any portion thereof, remaining unpaid of any assessment be payable forthwith and shall commit said amount, together with interest thereon from 30 days after the date the notice of the original assessment was sent if no portion has been added to a tax levy, or if a portion has been added to a tax levy, then with interest from October 1 of the year to which the last portion has been added, with their warrant therefor, to the collector for collection. If a part of a prior apportioned assessment is ordered to be payable forthwith, the payments shall be credited to the terminal or final years so as to reduce the period of payment.